Implacable
/-ˈpleɪ-/
adjective
Not able to be placated or appeased.
Impossible to prevent or stop; inexorable, unrelenting, unstoppable.
Adamant; immovable.
Synonyms: impacable, irreconcilable, unassuageable, unplacable, unpleasable, relentless, unremitting, unyielding